Changes in membrane lipid composition secondary to interferon-induced cell surface modifications are being examined. Fatty acid composition of cell surface lipids after interferon treatment was measured by chromatographic procedures, using a membrane impermeable chemical probe to covalently derivatize the cell surface aminophospholipids. In resting lymphocytes, an asymmetry of phosphatidylethanolamine molecular species was observed, with a more saturated pattern in the fatty acid composition of the external leaflet of the plasma membrane. Both Alpha- and Beta-interferon caused the increase of arachidonic acid, as well as of other polyunsaturated fatty acids in the lymphocyte's surface phospholipids, resulting in a more unsaturated pattern in their fatty acid composition. These changes may result in an alteration of the plasma membrane motional state and may modulate, by this mechanism, certain membrane-dependent cell functions, including cytotoxicity. The effect of interferon on the lipids of the NK-susceptible target cell K562 was also examined. The development of the interferon-induced alterations in lipid composition of K562 cells was time-dependent, and essentially biphasic, with an early decrease in saturated fatty acids, and a later increase in saturated fatty acids while polyunsaturated fatty acids were decreasing. The sensitivity of K562 cells to NK lysis was also modified by interferon pretreatment, in a time-dependent and biphasic manner. A short-term interferon pretreatment resulted in an increased sensitivity of these cells to lysis, while they became more resistant to lysis after long-term interferon pretreatment, their sensitivity correlating with the changes in lipid composition. These findings, documenting the involvement of cell membrane lipids in sensitivity of tumor cells to cellular cytotoxicity opens up possibilities in cancer treatment for manipulating tumor cells to make them more susceptible to elimination by the host's defenses.